There is a great need in the semiconductor industry for metrology equipment which can provide high resolution, nondestructive evaluation of product wafers as they pass through various fabrication stages. In recent years, a number of products have been developed for the nondestructive evaluation of semiconductor samples. One such product has been successfully marketed by the assignee herein under the trademark Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos. 4,634,290; 4,646,088; 5,854,710 and 5,074,669. The latter patents are incorporated herein by reference.
In the basic device described in the patents, an intensity modulated pump laser beam is focused on the sample surface for periodically exciting the sample. In the case of a semiconductor, thermal and plasma waves are generated in the sample which spread out from the pump beam spot. These waves reflect and scatter off various features and interact with various regions within the sample in a way which alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. Features and regions below the sample surface which alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
In the basic device, a second laser is provided for generating a probe beam of radiation. This probe beam is focused colinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of reflected probe beam. The photodetector generates an output signal which is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes which are synchronous with the pump beam modulation frequency. In the preferred embodiment, a lock-in detector is used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity (MOR) of the sample.
This system has the advantage that it is a non-contact, nondestructive technique which can be used on product wafers during processing. Using lasers for the pump and probe beams allows for very tight focusing, in the micron range, to permit measurements with high spatial resolution, a critical requirement for semiconductor inspection. The prior system has been used extensively in the past to monitor levels of ion doping in samples since the modulated optical reflectivity is dependent on ion dopant levels in the sample. This dependence is relatively linear for the low to mid-dose regimes (10.sup.11 to 10.sup.14 ions/cm.sup.2). At higher dopant concentrations, the MOR signal tends to become non-monotonic and further information is needed to fully analyze the sample.
One approach for dealing with the problem of monitoring samples with high dopant concentrations is to measure the DC reflectivity of both the pump and probe beams in addition to the modulated optical reflectivity signal carried on the probe beam. Using the DC reflectivity data at two wavelengths, some ambiguities in the measurement can often be resolved. The details of this approach are described in U.S. Pat. No. 5,074,669, cited above.
Semiconductor fabrication technology is increasing in complexity at a rapid pace. Various multilayer structures are being developed which makes testing more difficult. In addition, manufacturers are seeking to increase yields by fabricating chips on larger diameter wafers. As the diameter of the semiconductor wafers increases, the cost and value of each wafer increases. When using large, valuable and expensive wafers, it is no longer economically viable for manufacturers to rely on any forms of destructive testing methodologies. Therefore, there is a great need to provide equipment which can characterize complex structures with many unknowns or variables in a nondestructive manner.
Inspection problems also arise where metalized layers are deposited on semiconductors. If a typical metal layers is more than 100 angstroms thick, it will generally be opaque to more commonly used optical wavelengths. Therefore, equipment designed to monitor relatively transparent oxide layers cannot be effectively used to inspect metalized layers. Therefore, some new methodologies are required in order to inspect semiconductors with metalized layers. These layers can be formed from materials, such as aluminum, titanium, titanium nitride (TiN) and tungsten silicide (WSi).